Bipolar junction device

ABSTRACT

The present invention is related to a bipolar transistor in which the in-situ doped epitaxial Si or SiGe base layer is used instead of using an ion-implanted Si base, in order to achieve higher cutoff frequency. The SiGe base having the narrower energy bandgap than the Si emitter allows to enhance the current gain, the cutoff frequency(f T ), and the maximum oscillation frequency (f max ). The narrow bandgap SiGe base also allows to have higher base doping concentration. As a result, the intrinsic base resistance is lowered and the noise figure is thus lowered. Parasitic base resistance is also minimized by using a metallic silicide base ohmic electrode. The present invention is focused on low cost, high repeatability and reliability by simplifying the manufacturing process step.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a divisional of Ser. No. 09/469,395, filedDec. 22, 1999 and Applicant hereby claims priority thereto under 35 USC120. Furthermore. Applicant also claims priority under 35 USC 119 basedupon Korean patent application no. 10-1999-0037911, filed Sep. 7, 1999.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to bipolar junction elements like ajunction diode or a junction transistor and a method for manufacturingthereof. Especially, the present invention relates to a bipolartransistor and a method for manufacturing the bipolar transistor inwhich a silicon or a silicon-germanium(SiGe) film is used for a base.

The field of semiconductor with highly advanced technology can bedivided into the field of memory, the field of system integratedcircuit(IC) which is represented by application specific integratedcircuit (ASIC), the field of radio frequency integrated circuit (RFIC)which is essential in wireless communication, and the field of highspeed digital & analog integrated circuit for data process. Among thesefields of semiconductor, the present invention is especially related tothe semiconductor element having high maximum oscillationfrequency(f_(max)) and high cut-off frequency(f_(T)) which are relatedto the field of RFICs and the high speed digital & analog ICs. As welive in the era where wireless communication is widely used and whereits need is increasing explosively, the need of high frequency elementsis rising as the quality of communication improves and the markets ofhigher frequency band providing various communication services and largenumber of subscribers increase. Also, as the need of super speedinformation communication network increases in the contemporary society,researches and developments of a high speed transistor are carried onactively. At present, 10 Gbps integrated circuit using high speedtransistor about 60 GHz is developed in the usage of opticalcommunications and is at the stage of being commonly used. The siliconhomo bipolar junction transistor which includes the silicon base layerformed by ion-implanting a dopant has maximum f_(T) of 30 GHz at most.The heterojunction bipolar transistor(HBT) having an epitaxially grownSiGe base layer exhibits maximum f_(T) in the range of 50˜150 GHz andf_(max) of 50˜160 GHz.

The SiGe has a narrower energy bandgap(E_(g)) than that of the silicon.The energy bandgap difference(ΔE_(g)) between the silicon emitter andthe SiGe base enhances the current gain exponentially, and the f_(T) andthe f_(max) also increase compared to the homojunction bipolartransistor. Therefore, the impurity doping concentration into the SiGebase can be increased in the margin of exp (ΔE_(g)) without degradingthe current gain. The base resistance is lowered and consequently thenoise figure is lowered. Furthermore, it is possible for powerconsumption to be lowered because the bias current achieving the samef_(T) decreases. In comparison with the base of the transistor formed byion-implanting in the conventional method, the base formed by theepitaxial growth method can be much thinner to the degree of 200 Åapproximately and consequently the cut-off frequency increases. Sincethe Ge composition in the SiGe base from the emitter side to thecollector side ramps up linearly, the electrons transiting to the baseaccelerate. Therefore, the f_(T) and f_(max) can be further increased bygrading the Ge content. The SiGe HBT is fully process-compatible withsilicon devices. Furthermore, it allows to achieve higher f_(T) andf_(max) than 100 GHz using 0.8˜1 μm of photolithography. This meansthat, contrast to memory and ASIC getting highly scale-down to 0.18˜0.25μm, SiGe HBTs can be fabricated by recycling the out-of-date productionfacilities at the 0.8˜1 μm level. Therefore, it has good economic valuewith high output.

2. Description of the Prior Art

There are several registered patents showing related arts of SiGe HBTfrom IBM in the United States, NEC, Hitachi, and SONY in Japan, TEMIC inGermany, and Electronics and Telecommunications Research Institute(ETRI)in Korea. The structural characteristics and drawbacks of the relatedarts will be given in the followings.

First, the prior art of NEC in Japan is a kind of the super self-alignedNPN HBT. In this particular transistor, the base layer including SiGe isselectively grown in the device active region and each of thecollector-base and the emitter-base junctions is self-aligned. Themethod for manufacturing this super self-aligned will be describedherein below.

In FIG. 1a, a n+ type buried collector 11 is formed by ion-implanting n+type impurity (dopant) into a p− type silicon substrate 1. A collectorlayer 10 is deposited on the resulting structure. A collector sinker 13which connects the buried collector 11 and a collector semiconductorelectrode to be formed afterward is formed by implanting n+ typeimpurity ions into the region as shown in the figure. A trench is formedby etching the collector layer 10 and the substrate 1 in order toisolate the neighboring transistors electrically. The isolation trench71 is filled with an insulation material like boron phosphorous silicaglass (BPSG). Then, the surface of the isolation trench 71 is planarizedby a chemical-mechanical polishing (CMP) of the BSPG so that the surfaceof the isolation trench 71 becomes a same height with the surface of thecollector layer 10. Form a collector insulation layer 17 with siliconoxide layer, a base semiconductor electrode 23 with p+ type polysiliconfilm, and an emitter insulation layer 37 with silicon nitride film bysequentially depositing on the substrate 1 where the collector layer 10and the isolation trench 71 are formed. The collector insulation layer17 in the region which is planned to be the emitter is exposed byetching the emitter insulation layer 37 and the base semiconductorelectrode 23. Then, by depositing an insulation layer and etching itanisotropically, a first side-wall insulation layer 73 is formed at theinner side-wall of the base semiconductor electrode 23 and the emitterinsulation layer 37. Wet etch the exposed collector insulation layer 17in order to expose the collector layer 10 beneath the collectorinsulation layer 17. Even after the collector layer 10 is exposed,continue the wet etching to form an undercut 27 a to the pre-determineddepth beneath the base semiconductor electrode 23. The n type impurityions are added selectively to the intrinsic collector region 15 byion-implanting to the resulting structure in order to increase thecut-off frequency.

In FIG. 1b, a base layer 20 composed of an undoped SiGe, a p+ SiGelayer, and an undoped Si layer which is supposed to be the emitter 35later on is grown selectively on the exposed collector layer 10 andbeneath the exposed base semiconductor electrode 23 in the undercut 27a. Here, a base connecting part 27 b which is selectively depositedbeneath the base semiconductor electrode 23 is a poly-crystalline whilethe base layer 20 on the collector layer 10 is single crystalline. Asilicon film is further selectively grown thereon in order to make surethe connection between the base semiconductor electrode 23 and the baselayer 20. At this step, growth rate of the single crystalline siliconlayer on the base layer 20 is controlled to be much slower than that ofthe poly-crystalline base connecting part 27 b, so that the thicknessvariation of the undoped Si layer at the top of the base layer 20 isminimized. The second side-wall insulation layer 75 which covers thefirst side-wall insulation layer 73 and which contacts with a part ofthe base layer 20 is formed by depositing the insulation material like asilicon nitride film and by etching it anisotropically. Then, expose thecollector sinker 13 by partially opening the collector insulation layer17. An n type polysilicon layer is deposited on the resulting structure.Then it is patterned to form an emitter semiconductor electrode 33 onthe base layer 20 and a collector semiconductor electrode 13 a on thecollector sinker 13. Diffuse the impurity in the emitter semiconductorelectrode 33 into the undoped Si layer by thermal annealing, at theupper-most part of the base 20 to form an n type emitter 35. Theremaining part of the base 20 is the intrinsic base layer 25. As aresult, a super self-aligned transistor in which the collector-base partis self-aligned through the undercut and in which the emitter-base partis self aligned through the first and the second side-walls is formedwithout using masks.

Since the depth of the undercut 27 a is controlled by the wet etchingtime, it is difficult to get a good uniformity of the collector-basejunction parasitic capacitance which is determined by the depth. Whenthe base layer is selectively grown on the collector layer in theoxide-patterned wafer, the thickness of the base layer, the dopingconcentration in the base layer, and Ge concentration in the SiGe layervary according to the density and size of the exposed part of thecollector layer. This effect, so called loading effect, can lowerprocess stability and lead to non-uniform device performance within thewafer. In order to lower the loading effect, the growing pressure of thebase layer has to be decreased. However, the throughput is lowered inthis case. In this prior art, the base electrode is composed with p+polysilicon. As the parasitic resistance of the p+ polysilicon is biggerthan a metal, it obstructs the f_(max) being increased.

The second related art owned by the IBM in the U.S.A. is about a SiGeHBT with a titanium silicide layer which is used as an ohmic electrodeat the emitter, the base and the collector, as shown in FIG. 2, toreduce the parasitic resistances in the emitter, the base and thecollector. The method for manufacturing thereof will be briefly givenhereinafter.

In FIG. 2a, a buried collector 11 is formed by implanting an n+ typeimpurity into a p-type silicone substrate 1. A collector layer 10 isformed by depositing a silicon thereon. A trench is formed by etchingthe substrate 1 and the collector layer 10 for device isolation. Aninsulation layer is formed with insulation material like a silicon oxideat the inner, side of the trench. A polysilicon is filled into the restof the inner side of the trench and planarized by chemical-mechanicalpolishing. As a result, the polysilicon-filled trench isolation 71 isformed. A collector insulation layer (field oxide film) 17 is formed bythe recessed local oxidation of silicon (recessed LOCOS). In thismethod, the part excluding the active region of the collector layer 10is etched to a pre-determined depth and the part of the collector layerexcluding already etched part is thermally oxidized. In other words, thecollector insulation layer 17 is thermally oxidized. In other words, thecollector insulation layer 17 is formed only in the region of thecollector layer 10 excluding the collector sinker 13 which will beformed and the collector 15 on which emitter will be formed. An n+ typecollector sinker 13 is formed by implanting n type impurity ions into apart of the collector layer 10 using a photo-resistor and the collectorinsulation layer 37 as masks. On the resulting structure, a base layerincluding a p+ SiGe layer and undoped Si layer is grown at the top ofthe base layer which becomes an emitter at the diffusing of an n typeimpurity afterwards. At this step, mono-crystalline base layer to beused as a base 25 is deposited on the active collector 15 whilepoly-crystalline or amorphous base layer to be used as the basesemiconductor electrode 23 is deposited on the collector insulationlayer 17. The outer part of the base semiconductor electrode is etchedoff using a photo-resistor mask. An emitter insulation layer 37 like asilicon oxide is deposited. An emitter region is opened by eliminatingthe part corresponding to the active collector 15 and the base 25 of theemitter insulation layer 37 using a photo resistor mask.

In FIG. 2b, an emitter semiconductor electrode 33 is formed bydepositing and patterning an n+ polysilicon. A side-wall silicon oxidefilm 77 is formed at the outer side of the emitter semiconductorelectrode 33 by depositing silicon oxide material and etching itanisotropically. By further anisotropic oxide etching, the emitterinsulation layer 37 on the base semiconductor electrode 23 iseliminated.

Using a selective titanium silicide formation only on the siliconsurface, a collector ohmic electrode 19, a base ohmic electrode 29, andan emitter ohmic electrode 39 are formed simultaneously. In the presentdevice, the contact resistance and the base parasitic resistance isreduced by the metallic silicide ohmic electrodes 19, 29, 39 on thesemiconductor electrodes.

In this case, in order for the emitter metal electrode to be formed onthe emitter ohmic electrode, the size of the emitter ohmic electrodeshould be bigger than that of the emitter metal electrode. The narrowerthe emitter is, the higher the f_(T) and the f_(max) are. Therefore theedge of the emitter should be apart from the edge of the base ohmicelectrode to a distance L. In other words, the edge of the intrinsicbase beneath the emitter is apart from the edge of the base ohmicelectrode in the distance of L. Accordingly, occurrence of the parasiticresistance at the extrinsic base region with the length of L isunavoidable. In order to reduce the emitter contact resistance, emitterohmic electrode should be larger, but the length of L becomes larger.For this reason, there is difficulty in scaling down the device forhigher speed and lower power consumption. Extending the emitter ohmicelectrode to the outside of the active device region in order for theemitter metal electrode to be contacted thereon could be one way ofovercoming this difficulty. In this case the parasitic emitterresistance occurres at the extended part of the emitter ohmic electrode.

The titanium disilicide base ohmic electrode formed by sputtering thetitanium and by reacting it with the silicon underneath agglomeratesduring the silicide formation so, the titanium disilicide layer maypenetrate through the thin base layer and contact the collector layerand the cutoff frequency f_(T) may be lowered. For this reason, it isrisky to make the base layer thin unconditionally to achieve high f_(T).In another way, the base ohmic layer should be formed only in theoutside of the active device region at the expense of larger L. As aresult, the base parasitic resistance increases and the deviceperformance may be degraded.

In the third SiGe base bipolar device of FIG. 3 presented by TEMIC inGermany, a titanium disilicide layer 29 is used as a base ohmicelectrode and self-aligned with an emitter semiconductor electrode 33.The manufacturing method is described briefly in the followings.

In FIG. 3a, a buried collector 11 is formed by implanting n-typeimpurity ions into a p-type silicon substrate 1. A collector layer isdeposited thereon. A collector insulation layer 17 is formed by LOCOSprocess. The layer 17 is not formed in an active collector region 15 anda collector sinker 13. A base layer with p+ type SiGe and an emitterlayer with an n-type silicon are sequentially deposited thereon. A partof this layer deposited on the active collector region 15 is a singlecrystalline while the other part of it deposited on the collectorinsulation layer (field oxide layer) 17 is either a poly-crystalline oramorphous. An emitter insulation layer 37 with silicon oxide and asilicon nitride layer are sequentially deposited on the resultingstructure. Then, a masking film 91 is formed by patterning the siliconnitride layer using a photo-mask which covers the emitter region. Then-type silicon emitter layer outside the emitter region is converted toa p++ type first base semiconductor electrode film 21 a By implantingBF₂ ion thereon and annealing. The n-type silicon emitter layer insidethe emitter region, i.e. intrinsic emitter layer 35, remains unchanged.At the same time, the p+SiGe base layer outside the masking layerbecomes a p++ second base semiconductor electrode film 21 b while the p+SiGe base layer beneath the intrinsic emitter layer 35, i.e. intrinsicbase layer 25, remains unchanged. The B(boron) diffuses and thereby apedestal p++ region 27 is formed along with the inner periphery of theactive collector region 15, during the thermal annealing which followsthe BF₂ implantation.

In FIG. 3b, a first and a second base semiconductor electrode, 23 a and23 b respectively, are completely formed by patterning the films 21 aand 21 b using a photo-mask which defines the base electrode region. Afirst insulating side-wall 73 is formed at the side-walls of the nitridemasking film 91 and the base semiconductor electrodes consisting of 23 aand 23 b, by depositing a silicon oxidation layer and anisotropicallyetching. A part of emitter insulation layer 37 uncovered by the maskingfilm 91 and the first insulating side-wall are further eliminated by theanisotropic etching. Then, a titanium-silicide base ohmic electrode 29and collector ohmic electrode 19 are formed selectively only on theupper part of the exposed first semiconductor electrode 23 a and thecollector sinker 13. A protection layer 79 like a silicon oxide layer isdeposited on the resulting substrate. The protection layer 79 isplanarized by a chemical mechanical polishing(CMP) until the top surfaceof the masking film 91 is exposed.

The masking layer 91 is selectively etched off. A second side insulationlayer 75 is formed at the inner side of the first side insulation layer73. A part of the emitter insulation layer 37 is hence exposed and theneliminated by etching so that the n-type silicon emitter layer isopened. An emitter semiconductor electrode 33 is formed by depositingand patterning an n+ type poly-crystalline silicon using a photo-mask.An emitter ohmic electrode 39 is selectively formed with a titaniumdisilicide only on the emitter semiconductor electrode 33, by sputteringa titanium and thermal annealing. Metal contact windows of the baseohmic electrode 29 and the collector ohmic electrode 19 are opened bypatterning the protection layer 79 using a photo-mask. A base terminal81, an emitter terminal 83, and a collector terminal 85 are formedthereon by depositing and patterning the metal using a photo-mask.

In this method, at the thermal annealing step to activate the boron ionsimplanted in the base semiconductor electrode 23 a and 23 b, the boronions in the p+ SiGe intrinsic base layer 25 diffuses vertically into theadjacent silicon layers; the intrinsic emitter layer 35 and the activecollector layer 15. Therefore the base is thickened and cut-offfrequency is lowered accordingly. At the same time, the boron implantedin the base semiconductor electrode 23 a and 23 b laterally diffuses andcontacts the n type impurity which diffuses from the n+ typepoly-crystalline silicon emitter semiconductor electrode 33. In thiscase, an n+/p++ emitter/base junction is made and a tunneling current,in other words, a leakage current occurs.

Since a polishing rate of a silicon oxide is almost the same to that ofa silicon nitride in planarizing the protection layer 79 at the CMPstep, it is difficult to stop the polishing process of the protectionlayer 79 when the top surface of the masking film 91 is exposed. Thefirst insulating side-wall 73 may be over-polished and eliminate withthe protection layer 79. Therefore, the base ohmic electrode 29 maybe incontact with the emitter semiconductor electrode 33. In addition, as theprotection layer 79 outside the protruding part is also polished at thesame time, it could be eliminated and hence the emitter semiconductorelectrode 33 to be formed later could be in contact with the titaniumdisilicide base ohmic electrode 29. In order to avoid this kind ofdifficulty such as over-polishing of first insulating side-wall 73 andthe protection layer 79 outside the protruding part, it is desirable tomake the masking film 91 thicker. However, a thick silicon nitride filmputs on lots of stress to the substrate when it is deposited. Therefore,there is little process control margin.

The fourth related art, as shown in FIG. 4, is a SiGe base bipolartransistor proposed by ETRI in Korea. The method for manufacturingthereof will be described briefly in the followings.

In FIG. 4a, an n+ buried collector 11 is formed by ion-implanting n typeimpurities into a p− type silicon substrate 1. A collector layer isformed with n-type silicon thereon. A collector insulation layer 17 isformed by applying LOCOS process to the region in which a collectoractive region 15 and a collector sinker 13 are not formed. An n+collector sinker 13 is formed by implanting an n type impurity into acollector sinker region. A base layer 20 composed of three layers, anundoped SiGe, a p+ SiGe, and undoped Si layer from the bottom to thetop, is deposited on the entire substrate 1 where the active collectorregion 15 and the collector sinker 13 are formed. At this process step,the crystallinity of the base layer 20 deposited on the active collector15 and the collector sinker 13 is a single crystalline, while that onthe collector insulation layer (field oxidation film) 17 is apoly-crystalline or an amorphous. A masking film 91 covering thecollector sinker 13 and an active base region 25 within the activecollector region 15 is formed by depositing and patterning a siliconoxide film on the base layer. The BF₂ ions are implanted to the exposedbase layer using the masking film 91 as a mask. A heat treatment isfollowed in order to activate the implanted ions and re-crystallize thedamaged silicon layer by the implanting. At the same time, the implantedimpurity ions diffuse so that a p++ base semiconductor electrode film 21and a p++ region 27 at the edge of the collector 15 region are formed.Then the masking film 91 is eliminated. An amorphous base ohmicelectrode film 29 is deposited on the base layer by sputtering acomposite metal like TiSi_(2.6). A silicon oxide film 93 is furtherdeposited on the base ohmic electrode film 29. A part of the siliconoxide film 93 on the active base 25 is etched using a photo-mask whichopens the active base region 15. Then, the opened part of the base ohmicelectrode film 29 is wet etched accordingly by a HF-base chemicalsolution. An emitter insulation layer 37 like a silicon oxide layer isdeposited on the resulting substrate. The emitter insulation layer 37and the silicon oxide film 93 are etched using a photomask covering thebase electrode region. Then, the base ohmic electrode film 29 iswet-etched and the base layer 20 on collector insulation film 17 isdry-etched using the remaining emitter insulation layer 37 and theremaining silicon oxide film 93 as masks. A side-wall insulation layer77 is formed at each of the etched side-wall of the emitter insulationlayer 37, the silicon oxide film 93, the base ohmic electrode film 29,and the base semiconductor electrode 23. The emitter region is openedusing a photomask by etching the emitter insulation layer 37 andconsequently the active base layer 25 is exposed. An emittersemiconductor electrode 33 and a collector semiconductor electrode 13 aare simultaneously formed on the active base layer 25 and the collectorsinker 13 by depositing a polysilicon layer on the entire resultingsubstrate, implanting an n type impurity into the polysilicon layer, andpatterning the polysilicon layer using a photomask. A passivation layer79 is formed on the entire resulting substrate by depositing a siliconoxide. Then, by thermal annealing the substrate, the n type impurity inthe emitter semiconductor electrode 33 is diffused into the adjacentlayer so that the undoped Si layer, which is the upper layer of theactive base layer, is converted to an emitter 35. By patterning thepassivation layer 70 using a photomask, metal contact windows for theemitter, the base, and the collector are formed on the emittersemiconductor electrode 33, the base ohmic electrode 29, and collectorsemiconductor electrode 13 a, respectively. Then, a metal film includingTiW and A-1%Si is deposited and patterned using a phtomask definingmetal interconnection. Consequently, an emitter terminal 83, a baseterminal 81, and a collector terminal 85 are formed.

This related art also has drawbacks like other previously cited arts.First, when wet etching the amorphous TiSi_(2.6) base ohmic electrodefilm 29 using a photo-mask which opens the active base region 15, thewet etch rate of the amorphous TiSi_(2.6) base ohmic electrode film 29is different from that of the silicon oxide film 93. Consequently, theside-wall of the etched active base region becomes so uneven that a voidgeneration at the uneven side-wall is resulted when depositing theemitter insulation layer 37. When the polysilicon layer is deposited tobe the emitter semiconductor electrode 33 later on, the void is filledwith the polysilicon and therefore the emitter semiconductor electrode33 is in contact with the base ohmic electrode. In addition, since theetching process inevitably introduces bubbles at the reacting surface ofthe amorphous TiSi_(2.6) base ohmic electrode film 29, the bubblesprohibit the wet etchant from etching the amorphous TiSi_(2.6) film. Asa result, parts of the amorphous TiSi_(2.6) film covered with thebubbles are etched insufficiently that the amorphous TiSi_(2.6) residuesmay cause electrical contact between the emitter semiconductor electrode33 and the base ohmic electrode film 29.

Second, when wet-etching the base ohmic electrode film 29 outside of thebase electrode region using the emitter insulation layer 37 and thesilicon oxide film 93 as masks, the difference of the etch ratedifference between the silicon oxide and the amorphous TiSi_(2.6) filmresults in the uneven side-wall again. When cleaning the resulting waferusing a diluted HF, the emitter insulation layer 37, the silicon oxidefilm 93, and the amorphous TiSi_(2.6) film 29 are further etched so thatthe base electrode boundary become more unclear. That is, the baseelectrode formation process is not stable and therefore the resultingbase electrode becomes degraded electrically.

SUMMARY OF THE INVENTION

As forementioned, there are drawbacks and problems in the cited arts ofthe SiGe base bipolar transistor. In order to achieve higher cutofffrequency in a bipolar transistor, it is appropriate to have the baselayer as thin as possible. Since a titanium disilicide is likely toagglomerate, if the ultra thin base bipolar transistors is formed withthe titanium base ohmic electrode, the titanium disilicide is likely topermeate the thin base layer and thus contact the collector layer. Inorder to avoid this problem, the titanium base ohmic electrode is formedon the base layer outside the active collector region in the prior artof FIG. 2 at the expense of longer L as illustrated in FIG. 2a. Anotherway is suggested in FIG. 4 to overcome this drawback. When wet etchingthe base electrode formed with amorphous TiSi_(2.6), uniformity in theprocessing step cannot be guaranteed and an electrical contact is likelyto occur between the base and the emitter due to the amorphousTiSi_(2.6) residues at the etched surface of the active base region.

The object of the present invention is to provide an epitaxially grownSi- or SiGe-base bipolar transistor having a metallic silicide baseohmic layer to reduce the base electrode resistance and a manufacturingthe method thereof without aforementioned drawbacks. According to thepresent invention, a homojunction or heterojunction bipolar transistorcomprises of a semiconductor substrate having a buried collector; anactive collector and a collector sinker thereon which are divided by acollector insulation film; a base layer divided into an active baselayer on the active collector and a first base semiconductor electrodefilm on the collector insulation film; a second base semiconductorelectrode film selectively grown only on the first base semiconductorelectrode film by using a masking film; a metallic silicide base ohmicelectrode film selectively formed on the second base semiconductorelectrode film; an emitter insulation film isolating an emitter; anemitter formed on the active base layer; an emitter semiconductorelectrode formed on the emitter; a passivation film entirely coveringthe resulting structure; an emitter electrode, a base electrode, and acollector electrode formed on the emitter semiconductor electrode, thebase ohmic electrode, and the collector sinker, respectively.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1a and 1 b are cross-sectional views showing a super self-alignedheretojunction bipolar transistor (HBT) in which the base layer isformed by selective epitaxial growth of SiGe on the exposed siliconcollector surface.

FIGS. 2a and 2 b are cross-sectional views showing an SiGe HBT having atitanium disilicide base ohmic electrode and an SiGe base layer.

FIGS. 3a-3 c are cross-sectional views showing a self-aligned SiGe baseHBT having a titanium disilicide base ohmic electrode and an SiGe baselayer.

FIGS. 4a and 4 b are cross-sectional views showing a SiGe HBT having atitanium disilicide base ohmic electrode and an SiGe base layer.

FIGS. 5a to 5 g show sequential processing steps of manufacturinghomojunction or heterojunction bipolar transistor having a metallicsilicide base ohmic electrode and an Si- or SiGe-base layer according tothe preferred embodiment 1 of the present invention.

FIG. 6 is a cross-sectional view showing homojunction or heterojunctionbipolar transistor having an Si- or SiGe-base layer according to thepreferred embodiment 2 of the present invention.

FIGS. 7a to 7 c show the first example of sequential processing steps ofmanufacturing homojunction or heterojunction bipolar transistor havingan Si- or SiGe-base layer according to the preferred embodiment 3 of thepresent invention.

FIG. 8 is a cross-sectional view showing homojunction or heterojunctionbipolar transistor having an Si- or SiGe-base layer according to thesecond method shown in the preferred embodiment 3 of the presentinvention.

FIGS. 9a to 9 c show sequential processing steps of manufacturinghomojunction or heterojunction bipolar transistor having an Si- orSiGe-base layer according to the preferred embodiment 4 of the presentinvention.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

Referring to figures, the detailed methods for manufacturing a bipolartransistor will be explained, hereinafter according to preferredembodiments of the present invention.

Preferred Embodiment 1

Referring to FIGS. 5a and 5 g, method for manufacturing a bipolartransistor will be given in detail hereinafter according to the firstpreferred embodiment of the present invention.

An n+ buried collector 111 is formed by ion-implanting and diffusing ntype impurities like arsenic or phosphorous on the p-silicon substrate101, and a collector layer 110 is deposited thereon as shown in FIG. 5a.

A collector insulation layer (field oxidation film) 117 is formed on theregion excluding the region which will become a collector region 115 andcollector active region 115 a, and a collector sinker 113 by thermaloxidation like the LOCOS process. Using a photo-mask opening thecollector sinker 113, n type impurity like arsenic or phosphorous isimplanted to make the collector sinker heavily doped. Then the photomaskis stripped off an annealed to make the implanted impurity diffuse. Ann− type impurity like arsenic or phosphorous is implanted into an areaof intrinsic collector region 115 to form an active collector region 115a using a photo-mask Then the photomask is stripped off and thermalannealing is followed as shown in FIG. 5b.

A base layer 120 is deposited on the entire resulting substrate. Thebase layer 120 is a boron-doped silicon when a transistor is ahomojunction bipolar transistor. In heterojunction bipolartransistor(HBT), the base layer 120 is a multi-layer structurecomprising an undoped Si seed layer, an undoped SiGe film, a p+ SiGefilm, and an undoped Si film from the bottom to the top. The Si seedlayer enhances the uniformed nucleation of the SiGe film on thecollector insulating film so the thickness of the base layer 120 and theboron concentration and the germanium composition in the base layer 120are unified. In other words, the loading effect, in which the thicknessof the base layer, the doping concentration and the Ge(Germanium)composition in the base layer vary according to the size of an opened Siarea and the density of the opened Si pattern, does not occur. Inaddition, without the Si seed layer, the pressure at which the baselayer is deposited should be lowered to the lower deposition rate as itis appropriate to have higher deposition pressure for higher throughput.That is, with the Si seed layer, deposition pressure of the base layercould be higher for higher throughput. The undoped Si layer at the topwill be an emitter later on. The base layer 120 is patterned using aphotomask which covers the base electrode region, as shown in FIG. 5c.

A masking film 191 which covers an active base region 125 and thecollector sinker region 113 is formed by depositing and patterning witha photo-mask, as shown in FIG. 5d. The masking film comprises at leastone of a silicon oxide and a silicon nitride. The masking film dividesthe base layer 120 into two parts; one is the active base region 125 andthe other is a base semiconductor electrode region 123 a, as shown inFIG. 5d. BF₂ ions are implanted using the masking film 191 so that thebase layer 120 outside the masking film becomes a first basesemiconductor electrode 123 a with a higher boron doping concentration.Thermal annealing is followed and the implanted boron in the first basesemiconductor electrode diffuses into the edge side 127 of the activecollector 115.

Referring to FIG. 5e, an in-situ boron doped second base semiconductorelectrode 123 b is selectively deposited only on the first basesemiconductor electrode 123 a. The second base semiconductor electrode123 b comprises at least one from silicon, silicon-gernanium, andgermanium. Then, a base ohmic electrode 129 comprising a TiSi₂ and TiNis selectively formed only on the second base semiconductor electrode123 b by sputtering Ti and TiN sequentially and thermal annealing andwet etching. The base ohmic electrode 129 comprises at least one ofmetallic silicides such as TiSi₂, TiN, CoSi₂, PtSi₂, NiSi₂, FeSi₂,CuSi₂, WSi₂, etc. and metal as W, Cu, etc. The second base semiconductorelectrode is to prevent electrical contact between the metallic silicidebase ohmic electrode and the active collector region 115 a. Theelectrical contact may be caused if the ultra-thin base is agglomeratedwith the metallic silicide base ohmic electrode. In other words, theagglomerated silicide penetrates the ultra-thin base layer to contactthe collector region 115 a and consequently a Schottky junction isformed between the base and the collector. In this case, asymmetrybetween the base-collector and the emitter-base junctions results in anundesirable offset collector-emitter voltage and thereby deviceperformance degrades. As the base ohmic electrode 129 and the secondbase semiconductor electrode 123 b are not formed on the active baseregion 125, the cut-off frequency is enhanced as the thickness of theactive base region 125 is narrowed for there is no limit in thickness toprevent agglomeration.

Referring the FIG. 5f, an emitter insulation layer 137 is depositedentirely on the resulting substrate. Then, an emitter region is openedusing a photomask by etching the emitter insulation layer 137 and themasking film 191. An n+ polysilicon is formed and patterned using aphotomask which defines an emitter semiconductor electrode 133. Then,thermal annealing is followed. At this step, an n-type impurity in theemitter semiconductor electrode 133 diffuses to the adjacent active baselayer underneath, i.e. an intrinsic base region, and consequently theundoped Si film at the upper-most part of the base layer 125 isconverted to an emitter 135.

Referring to FIG. 5g, a passivation layer 179 with insulation materiallike silicon oxide or silicon nitride is deposited on the entireresulting substrate. Then, the opening of a contact window using aphoto-mask is followed to form a base contact window on the base ohmicelectrode 129, to form an emitter contact window on the emittersemiconductor electrode 133, and to form a collector contact window onthe collector sinker 113. The opening of the contact window etches thepassivation layer 179, emitter insulation layer 137, and the maskingfilm 191 which covers the collector sinker 113. After cleaning theresulting substrate using a diluted HF solution, a metal electrode layeris deposited and patterned suing a photomask so that a base terminal181, emitter terminal 183, and collector terminal 185 are formed asshown in FIG. 5g.

In case that the metallic silicide base ohmic electrode 129 isHF-soluble, the base ohmic electrode 129 can be etched during the diluteHF cleaning. In order to assure the connection between the base ohmicelectrode 129 and the base terminal 181, an additional metallic silicidelayer could be re-formed selectively on the exposed second basesemiconductor electrode within the base contact window, beforedepositing the metal electrode layer.

Preferred Embodiment 2

In this preferred embodiment in FIG. 6, the masking film 191 coveringthe active base 125 and the collector sinker 113 comprises two layers;which are a silicon oxide bottom layer 191 a and a silicon nitride toplayer 191 b. This is to avoid the damage on the base layer when openingthe emitter window by etching the emitter insulation layer 137 and themasking film 191. By way of the reactive ion etching (RIE) ratedifference between a silicon oxide and a silicon nitride, the RIE of theemitter insulation layer 137 stops at the silicon nitride layer 191 band then following another RIE of the exposed silicon nitride layer 191b stops at the silicon oxide bottom layer 191 a. The exposed siliconoxide bottom layer is eliminated by chemical wet etching withoutdamaging on the active base layer underneath.

Preferred Embodiment 3

In the preferred embodiment 1 of the present invention, the n typeimpurity in the heavily doped emitter semiconductor electrode with adoping concentration of ˜10²¹ cm⁻³ is diffused into the undoped Si layerat the upper part of the base layer and the n+ emitter is formed. At thesame time, the p type impurity in the p+ SiGe base with a dopingconcentration of ˜10¹⁹ cm⁻³ is also diffused into the undoped Si layer.As a result, the n+p emitter-base junction with a high junctioncapacitance is formed and consequently the cut-off frequency at the lowcollector current decreases.

In this preferred embodiment, a method of forming an emitter having alower emitter-base junction capacitance and higher cut-off frequency atlow collector current is presented. Details will be given hereinafterwith reference to FIGS. 7a to 7 c.

In FIG. 7a, the base 125 comprises an undoped Si seed layer, an undopedSiGe layer, and a p+ SiGe layer from the bottom to the top however, theundoped Si layer at the top of the SiGe base of the preferred embodiment1 is not included in this structure. The base 125 is exposed as theemitter region is opened by etching the emitter insulation layer 137 andthe masking film 191 after the step shown in FIG. 5e of the preferredembodiment 1.

In FIG. 7b, a phosphorous- or arsenic-doped single crystalline siliconemitter 135 a with a doping concentration lower than ˜10¹⁸ cm⁻³ isselectively grown only on the exposed base layer in the emitter region.

In FIG. 7c, an emitter semiconductor electrode 133 is formed bydepositing and patterning a phosphorous- or arsenic-doped polysilicon.

Another manufacturing method for the emitter 135 a and the emittersemiconductor electrode 133 according to the present embodiment can beapplied. After the emitter region is opened, an n-type silicon emitterlayer 135 a with a doping concentration lower than ˜10¹⁸ cm⁻³ and then-type emitter semiconductor electrode 133 with a doping concentrationof ˜10²¹ cm⁻³ are deposited sequentially on the entire substrate insteadof growing only the emitter layer 135 a selectively. Then the emitterlayer 135 a and the polysilicon emitter semiconductor electrode 133 arepatterned together. In this case, as shown in FIG. 8, the bipolartransistor structure is formed.

Preferred Embodiment 4

In this embodiment, unlike the way of forming the active collector asshown in the preferred embodiment 1, a different way of forming theactive collector, so called LOCOS process, is presented with referenceto FIGS. 9a to 9 c.

In FIG. 9a, the collector insulation layer 117 is deposited on theentire substrate in which the buried collector 111 is formed. The activecollector region 115 a and the collector sinker region 113 a are definedand exposed by etching the collector insulation layer 117 using aphotomask which opens the collector active region and collector sinkerregion.

In FIG. 9b, single crystalline silicon films 115 b and 113 b areselectively grown simultaneously only on the exposed silicon of thecollector region and collector sinker region. The single crystallinesilicon films 115 b and 113 b are overgrown than the collectorinsulation layer 117 in general, in order to completely fill the openedregions.

In FIG. 9c, the protruded part of the overgrown silicon films 115 b and113 b is planarized by a CMP. As a result, the collector insulationlayer 117, the active collector 115 and the collector sinker 113 areformed. Following processes hereinafter is the same as that in thepreferred embodiment 1.

While the present invention has been described with respect to certainpreferred embodiments, other modifications and variations may be madewithout departing from the scope of the present invention as set forthin the following claims.

The present invention provides homojunction bipolar transistor orheterojunction bipolar transistor using in-situ doped ultra thinepitaxial silicon or SiGe base layer. Therefore, the base layer of thebipolar transistor in the present invention is thinner than that formedby ion-implantation and as a result, the cut-off frequency(f_(T)) andmaximum oscillation frequency(f_(max)) are increased. In general, a SiGelayer is unlikely to be nucleated when it is grown on a field oxide. Dueto this fact, the SiGe base layer is likely to be selectively grown onlyon the active collector of the field oxide-patterned substrate. As aresult, it could be thicker than the critical thickness which is definedas a thickness limit of defect-free strained SiGe layer on a Si layer atdifferent Ge contents. Therefore, the strain relaxation resulting indislocation defect in the SiGe layer happens so that the SiGe HBT isfailed. In order to overcome this drawbacks the seed layer comprisingsilicon with a pre-determined thickness is firstly grown on theoxide-patterned substrate and then the SiGe base layer is grown thereon.In this case, the SiGe base layer becomes more uniformed in thicknessand the Ge composition together with doping concentration. In addition,the production yield increases by using this method of the presentinvention compared to using the conventional method in which the SiGebase layer is grown by using the selective epitaxial growth having alower growth rate and complicated process steps.

In particular, when forming the TiSi₂ base ohmic electrode selectivelynot only on the exposed SiGe base layer in the active collector regionbut also on the first base semiconductor electrode on the field oxide,the TiSi₂ may permeate the base layer and contact the active collectordue to the agglomeration of the TiSi₂. In this case, the performance ofthe device is degraded. In order to prevent this from happening, thein-situ doped second base semiconductor electrode is selectively grownonly on the region where the base ohmic electrode is to be formed.Safety and reliability of processing step is achieved. In order to lowerthe thermal budget of the manufacturing process, the heavily in-situdoped emitter semiconductor electrode is formed. As a result, thediffusion of the dopant in the SiGe base layer into the adjacent siliconcollector and silicon emitter is suppressed. Accordingly, the base isgrown as thin as possible and the emitter-base junction capacitance isdecreased so that lower noise and higher cutoff frequency could beachieved.

What is claimed is:
 1. A bipolar element comprising: a collector formedof a doped first type of semiconductor material; a base formed of asecond type semiconductor material in a base region of a base layer andcontacting the collector; a first base semiconductor electrode formed ofa doped second type of semiconductor material disposed on said baselayer and extending to the lateral sides of the base region; a maskinglayer defining the first base semiconductor electrode and an emitterregion by covering the base region; a second base semiconductorelectrode selectively formed on the first base semiconductor electrode;a base ohmic electrode formed on the second base semiconductorelectrode; an emitter insulating layer covering the base ohmic electrodeand portions of the masking layer; an emitter formed of the first typeof semiconductor material and contacting the base in an emitter regionwhich is defined by an opening in the emitter insulating layer and themasking layer and; an emitter semiconductor electrode formed of thedoped first type of semiconductor material and contacting the emitter.2. The bipolar element according to claim 1, wherein the base regionincludes: an in-situ doped base layer forming said base and; saidemitter, said emitter having been formed by an undoped semiconductormaterial deposited on the in-situ doped base layer.
 3. The bipolarelement according to claim 1, wherein the base region includes: a seedlayer; an undoped layer formed by an undoped silicon-germanium anddisposed on the seed layer; a doped layer formed by an in-situ dopedsilicon-germanium and disposed on the undoped layer and; an emitterlayer formed by an undoped semiconductor material disposed on the dopedlayer.
 4. The bipolar element according to claim 1, wherein the maskinglayer includees: a first masking layer formed of a silicon oxide and; asecond masking layer formed of a silicon nitride.
 5. The bipolar elementaccording to claim 1, wherein the emitter semiconductor electrodeincludes two contacting layers of the first type of doped semiconductormaterial disposed within the emitter region.
 6. The bipolar elementaccording to claim 1, wherein the emitter semiconductor electrodeincludes: a first emitter layer formed of a doped first type ofsemiconductor material and; and a second emitter layer formed of a moreheavily doped first type of semiconductor material.